Described below is a method for identifying an interference object in a scatter volume of an optical fire detector, which operates according to the scattered light principle. Also described below is an optical fire detector.
In optical smoke detectors it can happen that foreign objects, for example insects, trigger false alarms. It can therefore be expedient to use a number of sensor systems, which monitor adjacent volumes. If the sensor systems detect the same or a similar value for the scattered radiation, it can be assumed that the scatter results from a fairly homogeneous medium. It can thus be assumed that it is very probably smoke. If very different values are detected however, it must be a very non-homogeneous medium. It could be smoke occurring in swathes. However it is known from practical experience that smoke from a fire that normally occurs reaches the environment of a fire detector in a fairly uniform manner, so an upper value can be assumed for non-homogeneity. If the difference between the measured values is above this limit, it must be assumed that the scatter is caused for example by dust occurring spasmodically, by swathes of vapor, by an insect or by another object. It is then permissible to generate a fault message not an alarm or at least to delay the alarm.
Corresponding arrangements have been known for some time. WO 2005/069242 thus discloses the use of locationally distanced scatter volumes and the comparison of the light scattered in the volumes. The presence of an interference object is then concluded from the result of the comparison. It is then necessary to determine separate volumes, which requires a certain outlay and in particular cannot happen without structural elements which take up a certain amount of space.
An arrangement is even known, with which it is possible to use a mechanical arrangement to deflect the emitted light beam in a specific manner, thereby searching for interfering objects in the scatter volume in the manner of a searchlight. This arrangement also involves a high level of outlay and requires a lot of space.
In instances where a particularly small sensor volume is desired, for example in an alarm that is to be particularly flat so that it can be mounted unobtrusively below the ceiling, these known ways of defining separate scatter volumes are very obtrusive.